This application is in response to a request for application for studies of the use of 3-D conformal radiotherapy (3DCRT) in the definitive treatment of prostate cancer. UCSF is prepared to work with the NCI, the operations and quality assurance center and other successful applicants in designing and conducting clinical trials of 3DCRT. It is expected that initial studies will be phase I/II dose searching protocols and that after these are completed a phase III trial will be designed and carried out. The local control rate in advanced prostate cancer is relatively low and (based on recent PSA and biopsy data) patients with more limited stages have a lower local control than previously thought. It is possible that the higher doses that are possible with 3DCRT without an increase in morbidity could lead to improved cure rates. UCSF has all of the required computer hardware and an excellent 3-D treatment planning system which meets all of the requirements of the RFA. It is expected that in 1992 the UCSF group will treat 144 prostate patients with curative intent. Since 1989 the group has treated 215 prostate patients with conformal plans, 109 with a 2.5-D system and 106 with the full 3-D system. Extensive analyses have been carried out of our experience which have yielded information on the risk of nodal metastases , selection of patients for trials evaluating local control, definition of margins to be treated around the prostate and evaluation of set-up variation. We analyzed various conformal and non-conformal plans with dose volume histograms and identified a 6 field conformal technique using partial transmission blocks which compared to more conventional non-conformal techniques, allows a potential increase in dose on the order of 20%. The experience of the group in treating this number of patients with CT conformal plans, our experience in the RTOG 3DCRT committee and the studies mentioned above will assist the UCSF group in working closely under the RFA group to design optimal protocols.